Comparison
321 vs 347 Stainless Steel
Chemistry limits are from ASTM A240. The stabilizer specification is the defining difference: 321 requires Ti = 5×(C+N) min; 347 requires Nb = 10×C min, 1.0% max. Both base chemistries are otherwise 18Cr-8Ni austenitic stainless steels.
| Property | 321 / 321H (UNS S32100/S32109) | 347 / 347H (UNS S34700/S34709) |
|---|---|---|
| Chromium (Cr) | 17.0 – 19.0% | 17.0 – 19.0% |
| Nickel (Ni) | 9.0 – 12.0% | 9.0 – 13.0% |
| Stabilizer | Titanium (Ti) | Niobium (Nb) — also called Cb |
| Stabilizer Content | 5×(C+N) min, 0.70% max | 10×C min, 1.0% max |
| Carbon (C) max | 0.08% (321) / 0.04–0.10% (321H) | 0.08% (347) / 0.04–0.10% (347H) |
| Tensile Strength (min) | 515 MPa | 515 MPa |
| Yield Strength (min) | 205 MPa | 205 MPa |
| Max Continuous Service Temp | 816°C (1500°F) | 870°C (1600°F) |
| Carbide Stability Above 1000°C | TiC begins to dissolve | NbC remains stable |
| Recommended Filler Metal | ER321 | ER347 |
| Relative Material Cost | 1.20 – 1.30 (vs 304) | 1.25 – 1.40 (vs 304) |
Both 321 and 347 work by the same mechanism: a strong carbide-forming element is added to the alloy to tie up carbon before it can form chromium carbides. The difference is the element itself. Titanium has a carbide formation free energy of –162 kJ/mol at 1,000°C. Niobium has a free energy of –178 kJ/mol at the same temperature. The more negative value means NbC is thermodynamically more stable than TiC. In practice, this translates to NbC remaining stable up to approximately 1,150°C, while TiC begins to dissolve and coarsen above 1,000°C.
For pipe fittings and flanges, this matters when the metal temperature exceeds 900°C during upset or transient conditions. In a thermal well projecting into a 1,000°C gas stream, the outer sheath metal sees 925–975°C. TiC in 321 would begin to over-age, and the carbon would eventually re-enter solution, becoming available for chromium carbide formation during cooldown. NbC in 347 does not experience this dissolution, so the stabilization remains effective through unlimited thermal cycles. This is why 347 is the default for thermal wells, radiant superheater tubes, and furnace roll headers in steel mills.
At 800°C and below, 321 and 347 are effectively interchangeable. Both resist oxidation, maintain creep strength, and show no sensitization. Above 900°C, 347 pulls ahead. The NbC dispersion pins grain boundaries more effectively than TiC at these temperatures, retarding grain growth and maintaining higher creep rupture strength. Data from the ASME Boiler and Pressure Vessel Code Section II-D shows 347H having a 100,000-hour creep rupture strength of approximately 35 MPa at 750°C, versus 32 MPa for 321H. At 800°C, the gap widens to 22 MPa versus 18 MPa. For components designed for 200,000-hour life, that 15–20% strength advantage is decisive.
We see this in practice with our A213 TP347H tube bend orders. Boiler contractors specify 347H for reheaters and superheater outlet sections where steam temperatures are 580–620°C but the tube metal runs 50–80°C hotter due to heat flux. The 347H grade provides the margin needed for 30-year design life without tube replacement.
For single-pass or two-pass welds, ER321 and ER347 both perform well. The difference becomes clear on multi-pass welds in thick-wall flanges or heavy-wall pipe fittings. Each successive pass re-heats the prior passes into the HAZ sensitization range. With ER321, the TiC in the first pass can begin to coarsen or partially dissolve under the thermal cycle of the third or fourth pass. With ER347, NbC is more resistant to thermal cycling, so the stabilization remains intact through more passes.
Our welding procedure specification (WPS) for 347 flanges allows up to 6 passes without interpass temperature restriction beyond the standard 150°C limit. For 321 flanges, we limit to 4 passes or require a stabilizing anneal at 900°C between passes 4 and 5. This is not a theoretical concern — we have observed intergranular cracking in 321 multi-pass welds on 3" 300# flanges that were welded with 6 passes and then placed into 650°C service. The same geometry in 347 showed no cracking after 8,000 hours.
The niobium addition in 347 costs more than the titanium addition in 321. Ferro-niobium prices have traded between $35 and $55 per kg of contained Nb over the last 24 months, while ferro-titanium has ranged from $8 to $14 per kg of contained Ti. The higher stoichiometric requirement for Nb (10×C versus 5×(C+N) for Ti) also means more stabilizer mass is needed per ton of steel. The net result is that 347 typically costs 5–10% more than 321 at the mill level.
For a 12" 600# weld neck flange in 347 versus 321, the material cost difference in our 2025 quotes is approximately 8%. For a batch of 50 pieces, that is roughly $400–500 total. When the application is above 900°C, the 347 premium is negligible insurance. When the application is below 800°C, 321 is the more economical choice with no performance penalty.
Both grades form, bend, and machine similarly to 304. The stabilizer additions do not significantly alter work-hardening behavior. One caveat: 347 can form stringy chips during machining that adhere to tool inserts more aggressively than 321. We recommend using a slightly more positive rake angle (8° versus 5°) and a 15% higher cutting speed for 347 forgings to maintain tool life. For bending tube bends, both grades require the same bend radius (3×D for ≤2", 4×D for >2") and mandrel support to prevent wrinkling on the inner radius.
Our order book for 321 and 347 reflects distinct application patterns. In 2024–2025, we received three inquiries that illustrate the selection logic:
Our guidance to buyers: if the peak metal temperature is below 850°C and the weld is 4 passes or fewer, 321 is the cost-optimal choice. If the temperature exceeds 900°C, if multi-pass welding is required, or if the component is a thermal well or radiant tube, 347 is the safer specification.
Both are stabilized 18Cr-8Ni austenitic stainless steels. 321 uses titanium (Ti) as the stabilizer; 347 uses niobium (Nb, also called columbium Cb). Ti forms TiC; Nb forms NbC. NbC is thermodynamically more stable than TiC above 1,000°C, which gives 347 better performance in extreme high-temperature service. At temperatures below 850°C, the two grades are effectively interchangeable.
For 321: Ti = 5×(C+N) minimum, 0.70% maximum. For 347: Nb = 10×C minimum, 1.0% maximum. The 347 requirement is expressed as 10×C rather than 5×(C+N) because niobium also stabilizes against nitrogen but to a lesser degree than titanium. Our QC verifies these ratios on every MTC before accepting material.
No. 347 is better specifically above 900°C and in multi-pass welding. Below 850°C, 321 provides identical corrosion resistance, similar creep strength, and lower cost. Specifying 347 for a 600°C application is over-engineering and wastes 5–10% of the material budget. We have seen EPCs specify 347 for all high-temperature service regardless of actual temperature; our engineers routinely value-engineer these specs back to 321 when the metal temperature is below 850°C.
Yes, but use ER347 filler. The niobium in ER347 will stabilize the weld deposit regardless of whether the base metal contributes Ti or Nb. Do not use ER321 filler for a 321-to-347 joint — the Nb-free weld deposit will not be adequately stabilized against the carbon that diffuses from the 347 side during welding. Post-weld stabilizing anneal at 900°C is recommended for high-temperature service.
347H is the high-carbon variant (0.04–0.10% C) with the same Nb stabilization. The higher carbon provides better creep strength above 550°C by increasing the carbide volume fraction. 347 (0.08% C max) is the general-purpose grade. For boiler and reheater service, 347H is standard. For welded fittings in moderate temperatures, 347 is sufficient.
Thermal wells project into hot gas streams where the sheath metal can reach 925–1,050°C. At these temperatures, TiC in 321 begins to dissolve, and the stabilization is lost. NbC in 347 remains stable, so the sheath maintains immunity to sensitization through thermal cycling. We have supplied 347 thermal wells to gasification, incinerator, and furnace projects where 321 would have been inadequate.